A Tunable Molten-Salt Route for Scalable Synthesis of Ultrathin

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A Tunable Molten-Salt Route for Scalable Synthesis of Ultrathin Amorphous Carbon Nanosheets as HighPerformance Anode Materials for Lithium-Ion Batteries Yixian Wang, Wei Tian, Luhai Wang, Haoran Zhang, Jialiang Liu, Tingyue Peng, Lei Pan, Xiaobo Wang, and Mingbo Wu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b18313 • Publication Date (Web): 18 Jan 2018 Downloaded from http://pubs.acs.org on January 19, 2018

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ACS Applied Materials & Interfaces

A Tunable Molten-Salt Route for Scalable Synthesis of Ultrathin Amorphous Carbon Nanosheets as High-Performance Anode Materials for Lithium-Ion Batteries Yixian Wanga, Wei Tiana, Luhai Wangb, Haoran Zhangb, Jialiang Liua, Tingyue Penga, Lei Pana, Xiaobo Wanga, Mingbo Wua* a

State Key Laboratory of Heavy Oil Processing, College of Chemical Engineering,

China University of Petroleum (East China), Qingdao 266580, P. R. China. b

Petrochemical Research Institute, PetroChina Company Ltd., Beijing 102206, P. R.

China.

KEYWORDS: Amorphous carbon; Ultrathin carbon nanosheets; Petroleum asphalt; Molten salts; Lithium-ion batteries; Anode

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Abstract: Amorphous carbon is regarded as a promising alternative to commercial graphite as the lithium-ion battery anode due to its capability to reversibly store more lithium ions. However, the structural disorder with a large number of defects can lead to low electrical conductivity of the amorphous carbon, thus limiting its application for high power output. Herein, ultrathin amorphous carbon nanosheets were prepared from petroleum asphalt through tuning the carbonization temperature in a molten-salt medium. The amorphous nanostructure with expanded carbon interlayer spacing can provide substantial active sites for lithium storage while the two-dimensional (2D) morphology can facilitate fast electrical conductivity. As a result, the electrodes deliver a high reversible capacity, outstanding rate capability and superior cycling performance (579 and 396 mAh g-1 at 2 and 5 A g-1 after 900 cycles). Furthermore, full cells consisting of the carbon anodes coupled with LiMn2O4 cathodes exhibit high specific capacity (608 mAh g-1 at 50 mA g-1) and impressive cycling stability with slow capacity loss (0.16% per cycle at 200 mA g-1). The present study not only paves the way for industrial-scale synthesis of advanced carbon materials for lithium-ion batteries, but also deepens the fundamental understanding of the intrinsic mechanism of the molten-salt method.


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1. INTRODUCTION

In the past decades, lithium-ion batteries (LIBs) have achieved great success in powering portable electronic devices owing to their high energy density, long lifespan and light weight.1-4 With the ever-increasing market share of electric vehicles and hybrid electric vehicles, strict requirements are being imposed on electrode materials that are able to deliver high capacity and high energy output over long-time cycles.5-7 With respect to anodes, carbon materials, merited by their low cost, environmental friendliness and chemical stability, are widely regarded as the most promising electrode materials in LIBs.8-10 Unfortunately, as a commercial carbon anode, graphite suffers from the low theoretical capacity of 372 mAh g-1, which seriously limits its further application.11-13 Therefore, in the search for high-capacity anode materials, amorphous carbon has been emerging an alternative to graphite. By contrast, amorphous carbon possesses larger interlayer spacing and more disordered structure which can increase lithium storage capacity by intercalating more lithium between the graphene layers and absorbing extra lithium in the defects including pores and cavities.14 On the other hand, the barriers restricting both the power output and the capacity retention of a battery mostly involve the electrode pulverization and kinetics concerning intimate contact between the electrode and electrolyte, Li+ diffusion and electrical conductivity, which need to be imminently addressed for next-generation LIBs. Consequently, different kinds of carbonaceous materials including carbon nanospheres, carbon nanofibers, graphene and



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their composites have been carried out.15-19 In particular, two-dimensional (2D) carbon materials such as graphene and carbon nanosheets are attracting much attention because of their open structure with more active sites, sufficient interfacial contact with the electrolyte and fast electronic transport.20-25 However, typical synthetic processes associated with making these materials such as template methods and chemical vapor deposition (CVD) can be very complex and consume substantial hazardous chemicals, thus it is impending to develop a facile and green way for mass production of 2D amorphous carbon materials for LIB anode materials.26, 27

Petroleum asphalt, which originates from the distillation residue of crude oil and contains a large variety of different components such as phenols, polycyclic aromatic hydrocarbons, and heterocyclic compounds, can be a good carbon source due to its high aromatic content and high carbon yield.28 Nevertheless, direct pyrolysis of petroleum asphalt usually tends to form soft carbon that is highly graphitized, which is unfavorable to increase lithium storage. To solve this problem, Khosravi et al synthesized hard carbon (HC) through pyrolysis of pre-oxidized petroleum pitch with enhanced capacity, but the resultant HC delivered a very low initial Coulombic efficiency of 35%.29 Moreover, Li et al prepared ultrathin hollow carbon shell (HCS) using asphalt as the carbon source and the obtained HCS exhibited a stable capacity of 334 mA h g-1 at

a current density of 1 A

g-1.30 Although remarkable progress has been made using asphalt as feedstock to prepare high-performance carbon anodes over the years, it is still far from being industrialized


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and put on the market considering the relatively high technical requirements or complex procedures.

Taking the aforementioned problems into account, herein, we present a novel and scalable method for the synthesis of amorphous ultrathin carbon nanosheets through tuning the carbonization temperature of petroleum asphalt in molten metal chlorides. A eutectic composition of KCl and CaCl2 with a melting point of 600 oC was selected as the liquid medium because of its great abundance and chemical stability. Besides, these molten salts have been proved to effectively constrain the graphitization of petroleum asphalt. When used as LIB anodes, the amorphous carbon nanostructure with enlarged interlayer distance can provide substantial active sites while the unique 2D ultrathin nanosheets can facilitate fast Li-ion diffusion and electron transfer, thereby leading to an extraordinary cyclability and high rate capability. Furthermore, full cells consisting of the carbon anodes coupled with LiMn2O4 cathodes exhibit impressive cyclic stability, indicating a practical application potential of the petroleum-asphalt-derived amorphous ultrathin carbon nanosheets as anode materials for LIBs. More importantly, the evolution process of morphologies and nanostructures is systematically studied to help uncover the intrinsic mechanism behind the molten-salt method.

2. EXPERIMENTAL SECTION 2.1 Materials synthesis



Petroleum asphalt was provided by Sinopec Group as the carbon precursor. All other chemicals in the present work were of analytical grade and used without further purification. Typically, 0.5 g petroleum asphalt was added to 20 ml toluene followed by sonication for 20 min, forming a black solution. Then, 10 g eutectic salts of KCl/CaCl2 (2:1 by weight) were thoroughly mixed in an agate mortar before adding to the obtained solution. The above mixture was dried at 90 oC in an oil bath under stirring (160 rpm) to form a homogenous mixture and recovered the toluene. Subsequently, the brown power was heated to 800 oC (or 550 oC, 675 oC and 925 oC) at a heating rate of 5 oC min-1 and kept for 2 h in a temperature-programmed tube furnace under a continuous nitrogen flow. After cooled down to room temperature, the obtained product was washed with 1 M diluted hydrochloric acid and deionized water to remove salts and collected by vacuum filtration, followed by drying at 60 oC for 24 h. The petroleum-asphalt-derived molten-salt carbons (MSCs) prepared at temperature T were labeled as MSCT. To ascertain the influence of salts, bare petroleum asphalt carbon (PAC) was synthesized under the same condition of MSC800 without the addition of molten salts.

2.2 Materials characterization

Scanning electron microscopy (SEM) images were obtained on Hitachi S-4800. Transmission electron microscopy (TEM), high-resolution TEM (HRTEM) images and selected area electron diffraction (SAED) patterns were collected on JEM-2010 system at an accelerating voltage of 200 kV. Energy dispersive spectroscopy (EDS) was carried out


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on an EDAX system attached on the TEM system. The topographic data were recorded by atomic force microscopy (AFM, Shimadzu SPM-9700). X-ray diffraction (XRD) patterns were collected on an X'Pert PRO MPD diffractometer with Cu Kα radiation (λ=1.5406 Å). Raman spectra were performed on Renishaw RM2000 (512 nm laser). The Brunner-Emmet-Teller (BET) specific surface area and pore properties were measured by nitrogen adsorption and desorption on a Micromeritics ASAP 2020 analyzer. The surface elemental compositions were detected using an X-ray photoelectron spectroscopy (XPS, Thermo Scientific Escalab 250XI) with Al Kα radiation. Thermogravimetric analysis (TGA) was recorded on a Shimadzu TA-60Ws Thermal Analyzer in air or nitrogen at a ramp rate of 10 oC min-1.

2.3 Electrochemical measurements

The electrochemical performance was investigated using CR2032-type coin cells assembled in an argon-filled glove box (H2O < 0.1 ppm, O2 < 0.1 ppm). In a typical working electrode preparation, carbon products, acetylene black, and poly(vinylidene fluoride) (PVDF) were mixed with a mass ratio of 8:1:1 in N-methyl-2-pyrrolidinone (NMP) forming a homogeneous slurry. Then the resulting slurry was pasted onto Cu foil, followed by dring in a vacuum oven at 80 oC for 12 h. The loading amount of each electrode was kept at 0.6-0.8 mg cm-2. To assemble Li-ion half cells, lithium metal foil was used as the counter electrode, the electrolyte was comprised of 1 M LiPF6 in a mixture of ethylene carbonate (EC)/dimethyl carbonate (DMC) (1:1 v/v) while



polypropylene film was used as the separator. Cyclic voltammetry (CV) was tested on Autolab PGSTAT204 (Metrohm, Switzerland) at a scan rate of 0.1 mV s-1. The galvanostatic charge-discharge cycles and rate performances were investigated by a Land CT2001A battery test system (Wuhan, China) at a potential window of 0.01-3 V. Electrochemical impedance spectroscopy (EIS) was recorded at a frequency range of 100 kHz to 10 mHz with an AC amplitude of 5 mV on a CHI760D electrochemical workstation. As for Li-ion full cells, the cathodes were prepared by mixing the commercial LiMn2O4, acetylene black, and PVDF in the weight ratio of 8:1:1 with NMP, then the slurry was coated onto Al foil and dried at 80 oC for 12 h under vacuum. The typical loading amount was 4.5-5.5 mg cm-2. The cyclic voltammetry of LiMn2O4 half cells were tested at 0.05 mV s−1 from 3.2 to 4.3 V. All the above measurements were conducted at room temperature (25 oC).

3. RESULTS AND DISCUSSION The chemical element composition of petroleum asphalt is listed in Table S1. The high carbon content (86.27 wt%) demonstrates its carbon rich nature and it also possesses relatively high H with little N, S and O elements. Besides, thermogravimetric analysis (TGA) (Figure S1a) shows that its ash content is negligible with only 0.04 wt% residue at 1000 oC when tested in air. These results demonstrate that petroleum asphalt is an excellent carbon precursor for carbon materials. Moreover, the nature of KCl-CaCl2 eutectic salts was investigated using thermogravimetric and differential thermal analysis


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(TG-DTA). As shown in Figure S1b, the mixed salts exhibited good stability with only 1% and 14% weight loss at 800 oC and 1000 oC after the evaporation of absorbed water below 200 oC. The melting point is estimated to be 600 oC according to the DTA curve. After mixing the salts with petroleum asphalt, a sequence of samples were synthesized under different temperatures.



Figure 1. (a-d) SEM and (e-h) TEM images of MSC550 (a,e), MSC675 (b,f), MSC800 (c,g) and MSC925 (d,h).


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Figure 2. HRTEM images of MSC550 (a), MSC675 (b), MSC800 (c) and MSC925 (d). The insets of (a-d) are SAED patterns of each sample. Figure 1a-h show the scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of the as-obtained samples. It can be clearly seen that the morphologies of MSCs are quite different from the bulk structure of PAC (Figure S2a,b) and depend strongly on the carbonization temperature. More specifically, MSC550, synthesized at 550 oC, shows a typical porous carbon nanostructure with interconnected pores while MSC675 exhibits cheese-like morphology with irregular pits on the surface. With the temperature increasing to 800 and 925 oC, both MSC800 and MSC925 are dominated by folded ultrathin sheet-like structure. By contrast, MSC925 possesses thinner carbon nanosheets than MSC800 with an average thickness of 4.15 and



6.87 nm, respectively (Figure S3a-d). Besides, the thickness of MSC800 measured by atomic force microscopy (AFM) is ~5.8 nm (Figure S4), which is consistent with TEM results. Meanwhile, the energy dispersive spectroscopy (EDS) elemental mapping results (Figure S5) imply the homogenous distribution of C, O, N and S elements throughout the whole area of the carbon nanosheets. It is worth noting that the small amount of N and S signals mainly originate from the inherent nitrogen- and sulphur-containing heterocyclic compounds from petroleum asphalt and these heteroatoms could facilitate electron transfer and induce more defects.31 In the high-resolution TEM (HRTEM) images (Figure 2a-d), no apparent long-range ordered areas can be found in all MSCs, implying the amorphous structure of MSCs. Nevertheless, all samples but MSC550 contain some small domains of graphite micro-crystallites which develop randomly as the carbonization temperature rises. Also, the faint dispersing diffraction rings in selected area electron diffraction (SAED) patterns again evidence the amorphous nature of MSCs.32 Although PAC was prepared under the same temperature with MSC800, however, it has a long-range ordered margin composed of paralleled graphene sheets and a brighter diffraction ring (Figure S2c,d) which suggest that the introduction of molten salts can constrain the graphitization of MSCs.33


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Figure 3. (a) N2 adsorption and desorption isotherms, (b) pore size distribution based on DFT method and (c) digital images of 100 mg of MSCs and PAC samples. The porosity changes of MSCs are uncovered by Brunauer-Emmett-Teller (BET) analysis. Figure 3a depicts the nitrogen adsorption and desorption isotherms with a characteristic of IV type. The calculated BET specific surface areas (SSAs) and respective pore volume are listed in Table S2. Clearly, PAC only has a SSA of 4 m2 g-1, which illustrate the nonporous nature of the directly pyrolyzed asphalt. In contrast, the SSA of MSC550 is as high as 316 m2 g-1 with a pore volume of 0.24 cm3 g-1, confirming that the unmelted molten-salt can serve as a template to form porous carbon. However, there is a dramatic decrease of SSAs in MSC675, 800 and 925 (71, 104, and 93 m2 g-1, respectively) and micropore volume (0.01, 0.03, 0.01 cm3 g-1, respectively). In addition,



Figure 3b displays the pore size distribution of the obtained carbon products on the basis of the density functional theory (DFT) method and all MSCs show hierarchical micro-meso-macro characteristics, whereas a huge difference occurs in the micropore area of MSC550 while the meso-macro pore distributions almost remain unchanged compared with other MSCs. Considering both pore volume and SSAs, the structural change and pore collapse especially micropores caused by the movement of melting molten salts are evident. The overall BET analysis is in consistent with SEM and TEM results. In addition, the enlarged SSAs of MSCs also reflect a huge difference in the packing densities (Figure 3c).

Figure 4. Schematic illustration of the evolution process of MSCs.

From what has been discussed above, the as-obtained MSCs exhibit an obvious evolution of characteristics with the increase of carbonization temperature and a schematic of the forming process of ultrathin carbon nanosheets is presented in Figure 4.


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At first, PA was dissolved in toluene to uniformly mix with KCl-CaCl2 salts (Figure S6a). After the evaporation of the solvent, the mixture was carbonized under an inert atmosphere. According to the TGA curve in Figure S1a, the carbonization process of petroleum asphalt took place at around 400~520 oC, during which the notable weight loss was ascribed to the cleaving of side chains, the dehydrogenation of the aromatic hydrogen to produce the larger aromatic ring systems and the evaporation of volatile organic oil residues with smaller molecular weight.28, 34 When the temperature reached 550 oC, the carbon had already formed while salts still remained solid. So these unmelted salts could act as a hard template, leading to the formation of porous structure. After the temperature exceeded 600 oC, the eutectic salts began to melt and move slowly at 675 oC while pores collapsed and formed open structures. In the next stage, when the mixture gradually heated to 800 oC, the totally melted salts became an ionic liquid with drastic thermal motion, thus leading to a strong interaction of the as-formed carbon with ions. During the subsequent cooling process, the resultant was separated into two phases (Figure S6b) which was mainly because the recrystallization of salts pushed carbon out. Interestingly, Liu et al. reported the observation of exclusive dispersion of carbon in KCl in a eutectic LiCl-KCl system.35 However, the same result appeared in our KCl-CaCl2 system where carbon preferred to disperse in the KCl phase (Figure S6c). This phenomenon may arise from the sharp difference of the compatibility of MSCs with different salts of different polarity while the intrinsic mechanism deserves further studies. Consequently, the petroleum asphalt was completely converted to ultrathin carbon



nanosheets. Upon heating to a higher temperature, the resultant nanosheets became thinner and more shattered. Finally, MSCs were easily obtained after a simple washing step and most salts can be removed during this process according to the TGA curves in Figure S7. It is noteworthy that the carbon yield of MSCs was much higher than direct pyrolysis of petroleum asphalt at each temperature (Table S3) and this can be explained by the buffer effect of molten salts that act as barriers to prevent the escape of small organic molecules.

Figure 5. (a) XRD patterns and (b) Raman spectra of MSCs and PAC.

The microstructures of MSCs and PAC are further identified by X-ray diffraction (XRD) and Raman spectroscopy. As can be seen in Figure 5a, all samples exhibit two peaks at ~24° and ~43°, which are indexed to the (002) and (100) planes of amorphous carbon.36 The weak and broadened peaks of MSCs indicate a more disordered structure compared with PAC. The (002) peak usually moves to a higher angle and becomes narrower with increasing pyrolysis temperature, resulting in a continuously decreased


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interlayer space (d002).37 However, the d002 of MSCs calculated by Bragg equation (Table S2) first increases from 550 oC to 800 oC, and then drops at 925 oC. This abnormality is possibly because the strong interaction of ions in the liquid medium hinders the regular stack of sp2 carbon atoms to form graphite-like structures and thereby producing substantial topological defects. When the temperature exceeds 800 oC, the graphitization process will be dominant, thus leading to a shrinkage of interplanar distance. Therefore, MSC800 possesses the largest d002 of 0.37 nm, much wider than that of graphite (0.336 nm) and PAC (0.349 nm), which is in favor of the reversible storage of Li+. Raman spectra (Figure 5b) present two typical peaks of carbon materials at around 1350 and 1590 cm−1, corresponding to the disordered sp3 carbon (D band) and in-plane vibrational sp2 carbon (G band), respectively.38 Analogous to XRD results, the MSC800 sample exhibits higher intensity ratio of the D to G band (ID/IG) compared with the corresponding ratios for other MSCs and PAC (Table S2), indicating the numerous defects brought by molten salts within the ultrathin carbon nanosheets of MSC800, thereby providing additional active sites for lithium storage.39,

40

As shown in Figure S8, X-ray

photoelectron spectroscopy (XPS) indicates the surface elemental compositions of the obtained products. Consistent with EDS analysis, obvious C and O with weak N and S signals are observed in all samples without any impurity and their contents are summarized in Table S2.

As mentioned above, the characteristics including ultrathin sheet-like nanostructure,



enlarged SSAs, expanded interlayer space, and abundant defects endow MSCs especially MSC800 with great potential as the anode of LIBs. Therefore, the electrochemical properties of all samples were first evaluated in half cells, using Li metal as both counter and reference electrodes. Figure S9a-e depict the initial three consecutive cyclic voltammetry (CV) curves of each sample at a scan rate of 0.1 mV s-1 within a potential window of 0.01 to 3 V. All samples exhibit CV curves of typical carbonaceous anodes, as a sharp reduction peak (0.01 V) and two broad oxidation peaks (~0.2 and 1.2 V) are attributed to the lithium-driven reversible insertion into and extraction from carbon nanostructures (e.g. graphitic interlayers and nanovoids).41 For the first cycle, the cathodic peaks (around 0.7 and 1.7 V) are related to the electrolyte decomposition and irreversible reaction between lithium with functional groups on the carbon surface, thus forming a surface passive solid electrolyte interface (SEI) layer to prevent the electrolyte from further decomposition.42 However, an abnormal oxidation peak at ~2.3 V occurs in first anodic scan of MSC675 and MSC925 and will be discussed later. Furthermore, subsequent cyclic curves nearly overlap, indicating good repeatability. As shown in Figure S9f-j, the charge-discharge profiles of MSCs and PAC were measured at a current of 100 mA g−1 with the voltage range from 0.01 to 3.0 V. MSC800 delivers a higher initial discharge capacity of 1349 mAh g-1 than that of MSC925 (1293 mAh g-1), MSC675 (1228 mAh g-1), MSC550 (1198 mAh g-1) and PAC (744 mAh g-1). Meanwhile, an initial reversible charge capacity of 731 mAh g-1 is obtained in MSC800, giving an initial Coulombic efficiency (ICE) of 59.4%, which despite a lower ratio than 59.8% of


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MSC675 is the highest among PAC (54%), MSC550 (52%) and MSC925 (39%). The large initial capacity loss mainly originates from the inevitable formation of SEI films on the surface of the electrode and irreversible Li+ storage in some special positions such as in the vicinity of residual H atoms and defects.43, 44 As mentioned in CV analysis, a small plateau around 2.3 V is simultaneously found in the first galvanostatic charge curve for both MSC675 and MSC925, which may relate to Li+ extraction from the interfacial storage sites.45 In the next few cycles, the plateau at ~2.3 V disappears and the overall Coulombic efficiencies gradually improve to near 100%.

Figure 6. (a) Cycling performance at a current density of 1 A g-1 of MSCs and PAC, (b) long cycling stability at current densities of 2 and 5 A g-1 of MSC800, (c) rate performance of MSCs and PAC, and (d) the comparison of the rate performance of MSC800 with those previously reported anode materials. Figure 6a shows the cycling stability of MSCs and PAC electrodes at 1 A g-1. After



initial five cycles` preactivation at a low current density of 100 mA g-1, PAC exhibits a remarkably inferior capacity of 237 mAh g-1 after 700 cycles compared with MSCs, demonstrating that the molten salts can make a big difference to the enhanced electrochemical performance of MSCs. A noteworthy feature of the MSC electrodes is that the capacities gradually increase with deep cycling due to the further activation process of the electrode. And this process is quite common in carbon anodes, which is supposed to result from the gradual access of more electrolyte into the internal space of active materials.16, 46-48 By contrast, MSC800 retains a higher capacity of 680 mAh g-1 than that of MSC675 (583 mAh g-1), MSC925 (486 mAh g-1) and MSC550 (428 mAh g-1). Meanwhile, the MSC800 anode also delivers the highest capacity among the as-prepared samples with 599 mAh g-1 after 100 cycles at a low current density of 200 mA g-1 (Figure S10). In order to test its high-rate cyclability, the MSC800 electrode was further cycled at higher current densities (Figure 6b). Consequently, high reversible capacities of 579 and 396 mAh g-1 can be obtained after 900 cycles at 2 and 5 A g-1, respectively. Coulombic efficiencies in both measurements have maintained nearly 100%, regardless of current densities. Such an excellent cyclability is superior to a large number of previous results of the carbonaceous electrodes and also competitive with those heteroatom-doped carbon anodes (Table S4). Furthermore, the rate capability of MSC800 was investigated under various discharge/charge current densities from 100 mA g-1 to 10 A g-1 compared with the rest MSCs and PAC (Figure 6c). Expectedly, MSC800 shows the best rate performance with 697, 540, 447, 399, 358 and 315 mAh g-1 at current


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densities of 100 mA g-1, 200 mA g-1, 500 mA g-1, 1 A g-1, 2 A g-1 and 5 A g-1, respectively. Remarkably, the electrode can still exhibit a reversible discharge capacity of 283 mAh g-1 even at a high current density of 10 A g-1, corresponding to 40.6 % retention of the capacity at 100 mA g−1, although the current density is 100 times higher. Additionally, when the current was abruptly switched back to 100 mA g−1, the reversible capacity of MSC800 can recover to over 670 mAh g−1, giving a high capacity retention up to 96.1 % of the initial capacity. Other MSCs, those prepared at other carbonization temperatures, all deliver better rate capabilities than PAC. Figure 6d illustrates the comparisons in terms of rate capabilities of the MSC800 electrode with other reported electrodes, from which we can conclude that the rate performance of MSC800 is the best among those carbon anodes for LIBs.8, 15, 42, 49-54 The electrochemical performances of all samples are summarized in Table S5.

Figure 7. (a) Nyquist plots of MSCs and PAC with insets showing the equivalent circuit and the amplification of low-frequency area, (b) the relationship between Z’ and ω-1/2 in the low-frequency region.



To gain an insight into the superiority of the MSC800 electrode, electrochemical impedance spectroscopy (EIS) was employed to further study the differences of the electrodes’ performances. As can been seen from Figure 7a, the Nyquist plots of the samples are composed of a semicircle in the high-frequency area and an oblique line in the low-frequency region.51 After fitting with an equivalent circuit, the simulated data are listed in Table S5 where Re represents the resistance between the electrolyte and the electrode and Rct signifies the charge-transfer resistance. Notably, both Re and Rct values of MSC800 are smaller than its counterparts, suggesting the good contact of the electrode with the electrolyte as well as the current collector, and the effectively enhanced kinetics of charge-transfer inside the electrode, respectively.

Moreover, the Warburg impedance in low-frequency region of Nyquist plots can be used to calculate the lithium diffusion coefficient (DLi+) by applying the equation listed below,

= 2

where R is the gas constant, T is the absolute temperature, A is the surface area of the electrode, n is the number of electrons per molecule attending oxidization, F is the Faraday constant, C is the concentration of Li+, and σ is the Warburg factor which can be calculated according to the following relationship.55, 56

= + + ⁄ ACS Paragon Plus Environment

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As shown in Figure 7b, σ is the slope of the fitted curve between and ⁄ , and the calculated diffusion coefficients are shown in Table S5, where MSC800 still keeps the highest record of 1.04×10-12 cm2 s-1 among all the samples followed by MSC925 (9.56×10-13 cm2 s-1), MSC675 (1.39×10-13 cm2 s-1), MSC550 (6.65×10-15 cm2 s-1) and PAC (6.03×10-16 cm2 s-1). Moreover, the diffusion coefficients of MSC800 and MSC925 are several magnitudes larger than MSC500 and PAC, implicating the fast conductivity improved by 2D carbon nanosheets as well as the relative high graphitization degree.

Figure 8. Rate and cycling performances of MSC800/LiMn2O4 full-cell (inset is a demonstration of an array of 19 LEDs lighted by one MSC800/LiMn2O4 full cell). In views of these excellent performances of the MSC800 electrode in half cells, the MSC800 anode was assembled in full cells coupled with LiMn2O4 cathode to further explore the practical application in LIBs. As shown in Figure S11a, the commercial LiMn2O4 cathode is good crystallized and delivers an initial discharge capacity of 108 mAh g-1, with an ICE of 84% at 100 mA g-1 (Figure S11b). According to Figure S11c and S11d, the typical discharge plateaus of LiMn2O4 are 4.1 and 3.9 V, while its charge



plateaus are located at 4.05 and 4.15 V. Figure 8 shows the rate capability of MSC800/LiMn2O4 full-cell tested between 2.5 and 4.2 V, which delivers a discharge capacity of 608 mAh g-1 for 50 mA g-1, 439 mAh g-1 for 100 mA g-1, 356 mAh g-1 for 200 mA g-1, 280 mAh g-1 for 500 mA g-1, 218 mAh g-1 for 1 A g-1, and retains a capacity of 471 mAh g-1 after the current turns back to 50 mA g-1. Figure S12 displays the corresponding galvanostatic profiles of the full-cell at a series of rates. After the rate test, the cell was continuously cycled to investigate its cycling stability, it can be seen that a high reversible capacity of 212 mAh g-1 can still be maintained after 200 cycles at 200 mA g-1, with an average Coulombic efficiency of 99% and a capacity decay ratio of 0.16% per cycle. Although the MSC800/LiMn2O4 full cell exhibits a relative inferior performance compared with MSC800 half-cell, which is related to the capacity fade as well as the formation of SEI on both anode and cathode, it still holds great promise for practical energy storage devices due to its ability to deliver much larger reversible capacity than graphite (Figure 6). Furthermore, the overall performance of such a configuration can be improved by a delicate designed cathode. As a demonstration, an array of commercial light-emitting diodes (LEDs) was lighted using only one CR2032-type MSC800/LiMn2O4 full cell (Figure 8). The above results evidently manifest that the MSC800 anode exhibits high specific capacity, outstanding cyclability and excellent rate performance, which can be attributed to the following merits: (1) the 2D nanostructure is in favor of fast electrical conductivity


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while the ultrathin nanosheets can shorten ionic diffusion path, thereby building a highly conductive network for electronic and ionic transport; (2) the amorphous carbon with expanded interlayer distance and substantial defects can serve as the reservoir for sufficient lithium storage; (3) the large SSA facilitates the good electrolyte wettability and large exposure of active sites; (4) the robust carbon skeleton can mitigate the structural stress caused by volume expansion during Li+ insertion/extraction and protect the

electrode

from

pulverization.

Consequently,

MSC800

shows

outstanding

electrochemical performance.

4. CONCLUSION Ultrathin amorphous carbon nanosheets have been successfully prepared through carefully tuning the carbonization temperature of petroleum asphalt in a molten-salt medium. Meanwhile, the evolution process to form the 2D carbon nanomaterial is presented to help understand the molten-salt mechanism for future studies. With an optimum design, MSC800 exhibits a high specific capacity, standout rate capability and remarkable cyclic performance. In the half cell of LIBs, the material yields a capacity of 579 and 396 mAh g-1 at 2 and 5 A g-1 after 900 cycles. Moreover, MSC800/LiMn2O4 full cell exhibits a reversible capacity of 608 mAh g-1 at 50 mA g-1 and retains as much as 212 mAh g-1 at 200 mA g-1 after 200 cycles. This simple synthetic method not only provides a new way for scalable production of high-performance anode materials for LIBs, but also can be extended to prepare other functionalized 2D carbon materials with abundant



resources including biomass and coal pitch for advanced nanotechnologies.

ASSOCIATED CONTENT

Supporting Information.

The Supporting Information is available free of charge on the ACS Publications website at DOI: Elemental composition of petroleum asphalt; TG curves of petroleum asphalt and molten salts; SEM, TEM, HRTEM and SAED images of PAC; Thickness distribution of MSC800 and MSC900; AFM image of MSC800; EDS elemental mapping of MSC800; XRD patterns; XPS spectra of MSCs and PAC; electrochemical performances; summary tables of the characterization and electrochemical measurements results; the table of the comparison of cycling stability between this work and the previous reports.

AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected].

ORCID Mingbo Wu: 0000-0003-0048-778X

Notes The authors declare no competing financial interest.


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ACKNOWLEDGEMENTS This work was financially supported by the National Natural Science Foundation of China (Nos. 51372277, 51572296, U1662113); the Fundamental Research Fund for the Central Universities (Nos. 14CX02127A, 15CX08005A); the Joint Technology Development Project of CNPC (No. PRIKY16068).

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TOC Graphic


A Tunable Molten-Salt Route for Scalable Synthesis of Ultrathin

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